The present invention relates to switches for switching high electrical currents into electrical loads. More particularly, the present invention relates to explosively actuated switches.
Many types of explosively actuated switches have been proposed for various applications. Explosively actuated switches typically perform their switching functions very quickly. Such switches are thus utilized in applications where switching must be accomplished rapidly. A particular application of explosive switches is where a switch must carry extremely high currents and yet still switch quickly. For example, explosive switches may be utilized with large inductive energy storage systems, such as may be utilized to power electromagnetic railguns. In such applications, the switch is initially closed to provide high current charging of an inductor over an extended period of time, and then is exploded open to commute the current to the load within a relatively short time interval. High power explosive switches also have utility in other applications, such as, for example, actuating laser flashlamps and as circuit interrupts in the electric utility industry.
Inductive energy storage systems typically include a primary energy source, such as a homopolar generator, an inductor, and a primary opening switch element. Heretofore, inductive energy storage systems have been utilized as high voltage pulse generators and, more recently as the power source in railgun accelerators. The accelerating force in a railgun is obtained by the interaction of the current in a driven armature with the magnetic field produced by the current in the rails, with the armature and the rails being connected in series. The rate at which an opening switch delivers current into the load will effect the mechanical jerk seen by a projectile and its driving armature and will also effect the acceleration ratio of the gun. Furthermore, the current waveform seen by the railgun affects the stability of the armatures.
Prior art explosive switches suitable for carrying high current loads have typically utilized solid conductive plates. These prior art switches completely sever the conductive plate with explosive charges, thus, interrupting the circuit. In addition, the explosive gas flows over the metal vapor arc and raises the dielectric constant of the gas-metal vapor mixture as well as cooling the arc. Usually the switching waveforms associated with explosive switches produce current rise times of 125 microseconds or less. Although this produces desirable acceleration rates within a railgun, the resulting stresses on a projectile may be too high. Further, conventional explosive switch systems generally control the switching characteristics by increasing the inductance of the load, which results in a subsequent loss of efficiency. Therefore, it is desirable to produce more controlled switching characteristics from an explosive switch and to produce a slower current rise without adjusting the load parameters.
U.S. Pat. No. 4,571,468 to Weldon discloses a fast acting explosive switch. Weldon recognizes that a relatively slow controllable switching characteristic may be desirable. As typical in most explosive switches though, in Weldon the explosive force is applied such that it completely extends across the conductor. Thus, there is no suggestion in Weldon that the conductor may be only partially severed by the explosive forces. The switching characteristics contemplated in Weldon are, therefore, a function of the explosive pressure rise time and the resulting gap that is contemplated to extend completely across a conductor. Weldon addresses switching times of at least one millisecond. It is desirable to make switches slower than conventional explosive switches yet faster than Weldon. Weldon does not contemplate a region designed to thermally break down. It is desirable to be able to more accurately control the switching characteristics, in particular a load current rise time, than is available solely from adjusting pressure forces and the gap width geometries as in Weldon.
U.S. Pat. No. 4,538,133 to Pflanz discloses an auxiliary fuse placed in series with a parallel combination of an explosive fuse and a main fuse. When this switch system is exposed to a high current fault above a pre-determined level, the auxiliary fuse melts and the resulting voltage increase across the auxiliary fuse is used to detonate the explosive fuse. When the explosive fuse is blown, current flows through the main fuse which then melts to provide the final current interruption. U.S. Pat. No. 4,479,105 to Banes discloses a switch similar to Pflanz that incorporates an explosive fuse in parallel with a nonexplosive fuse.
Both of these patents disclose complicated switches that require at least two separate fuses placed in parallel and a connection means to connect these fuses. For example, Pflanz uses additional connecting straps 42a and 42b and associated bolts to connect a nonexplosive fuse to the explosive fuse. A simplified one conductor switch is desirable so that reliability and space requirements may be improved and additional connection problems and costs may be eliminated. Furthermore as noted in Banes, the function of such nonexplosive parallel fuses is to divert current from the main fuse in order to extinguish arcs. Also, these switches concentrate on quickly interrupting current rather than quickly interrupting current in a controlled manner to obtain a specific current characteristic and waveform.
It has now been discovered that slowing the switching current and controlling the switching waveform from an explosive switch produces desirable results in an inductive energy storage system. Therefore, it is desirable to have an explosive fuse that has controlled switching characteristics. Furthermore, the switching characteristics should enable current to be switched within hundreds of microseconds. In addition, it is desirable that such an explosive switch be incorporated into a simple one conductor system.